1. Field of the Invention
The invention relates to a control system for controlling input power to hydraulic pumps of a hydraulic driving system including a prime mover and a plurality of variable displacement hydraulic pumps driven by the prime mover and, more particularly, to a control system for controlling input power to hydraulic pumps of the hydraulic driving system in which the discharge rate of the variable displacement hydraulic pumps driven by the prime mover are controlled by means of solenoid valves by detecting tilting angles of the swash plates of the pumps and the discharge pressure of the pumps.
2. Description of the Prior Art
Heretofore, control systems of the type described hereinabove are known, for example, the control system disclosed in Japanese Patent Application No. 55-14049 of which corresponding patent applications are U.S. Ser. No. 387,884 (granted as U.S. Pat. No. 4,606,313), EPC Patent Application No. 81902759.0 and Korea Patent Application No. 3829/1981. This control system is adapted to control the input power to the pumps in such a manner that a difference in the rotational speed between an object rotational speed of the engine determined by the operation amount of the accelerator and the actual output rotational speed, namely, a deviation of the rotational speed is detected and object values of the tilting angle of the swash plates of the hydraulic pumps are calculated from the deviation of the rotational speed and the discharge pressure of the hydraulic pump itself so that the input torques are decreased as the deviation of the rotational speed is increased and then on the basis of the object values, the input powers to the respective hydraulic pumps are controlled.
This known control system is designed to control the power input torque of each pump in accordance with the discharge pressure of the pump itself. In the known system having two variable displacement hydraulic pumps, for example, in order that one of the pumps can make full use of the power of the engine while the load on the other pump is substantially zero, it is necessary to set the input torque of each pump to be substantially the same as the maximum output torque of the engine.
In this conventional control system, when large loads are simultaneously applied to the two pumps by operation of their control levers, a torque which is twice the engine output torque is applied to the engine, so that the engine speed is decreased and the input torques of the pumps are limited on the basis of the deviation of the rotational speed to balance the output torque of the engine.
However, because of the large inertia of a fly-wheel which is provided in the engine for the purpose of suppressing the fluctuation in the engine speed due to intermittent explosion occurring in the cylinders of the engine, there is a time lag which inevitably occurs from the moment at which the large torque exceeding the instant engine output torque is applied to the engine till the moment at which the rotational speed thereof is decreased. The time lag is caused also when the rotational speed is increased due to a reduction in the load applied to the variable displacement hydraulic pumps.
Due to this time lag between the changes in the torque (load) and the rotational speed, the conventional system has suffered from hunting.
Further, in the above system, when the control lever for one of the pumps is operated so as to apply a large load thereto while the other pump is operating under a certain discharge rate, a total load exceeding the instant output torque of the engine might be applied to the engine, so that the engine speed would be decreased. In consequence, the input torque of to the hydraulic pumps are limited in accordance with the increased deviation of the rotational speed to balance with the engine output torque. In consequence, the discharge rate of the variable displacement hydraulic pump which has been operated under the certain discharge rate is decreased. Thus, the discharge rate of one the pumps is undesirably changed as a result of a change in the discharge rate of the other pump. That is to say, the discharge rates of both pumps cannot be controlled independently.
It would be possible to prevent the hunting and to control the discharge rates independently thereby stabilizing the control, if the maximum input torque of each variable displacement hydraulic pump is designed to be about one-half of the engine output torque. In such a case, however, each pump can make use only half the output of the engine when the load on the other pump is substantially zero.
These problems are experienced also when three or more pumps are employed.